A safety factor is typically specified to protect against a
particular integrity-threatening mechanism, and different
safety factors can be used against different such mechanisms. Hence, one safety factor may be required for
resistance to plastic deformation and another for fatigue
resistance. As already indicated, a safety factor is most
commonly expressed as the ratio between a measure of the
maximal load not leading to the specified type of failure
and a corresponding measure of the applied load. In some
cases it may instead be expressed as the ratio between the
estimated design life and the actual service life.